Why do we fit a 10mm cable for showers?

[Bellendian]

But remember, we only work to 230v, most supplies I come across are still nearer 240...…...I'll bet them Brexit burgers aren't too bothered about our small discrepancies, though.

Indeed, but 240 is still within the permitted voltage range of a 230V nominal supply, so you'd like to think that equipment designed to operate on a 230V supply, with design calculations using the 230V nominal figures, would be perfectly capable of safe operation at the worst case scenario within the permitted voltage range.
The actual supply voltage should not invalidate calcs carried out based on 230V provided it is within the range permitted by ESQR.

 

[FatAlan]

Perhaps 10mm Is the safer option to cover future installs with bigger loads ... or indicative of a previous heavier rated shower?

 

[ipf]

Indeed, but 240 is still within the permitted voltage range of a 230V nominal supply, so you'd like to think that equipment designed to operate on a 230V supply, with design calculations using the 230V nominal figures, would be perfectly capable of safe operation at the worst case scenario within the permitted voltage range.
The actual supply voltage should not invalidate calcs carried out based on 230V provided it is within the range permitted by ESQR.

Nowt to do with it, mate. Just a point in replying to #16.